Resistance trimmer knob



y 1962 H. D. ZUNKER ET AL 3,032,734

RESISTANCE TRIMMER KNOB Filed July 5, 1960 INVENTORS,

Harold D. Zunker BY John Hocf Attorney United States Patent 3,032,734 RESISTANCE TRIMMER KNOB Harold D. Zunker and John Hoch, Milwaukee, Wis, assignors toGlobe-Union Inc., Milwaukee, Wis., a corporation of Delaware 1 Filed July 5, 1960, Ser. No. 40,596 4 Claims. (Cl. 338-163) This invention relates to improvements in adjusting means for electrical trimmers, particularly such trimmers as are initially adjusted to balance a circuit and then left in such adjusted state.

Trimmers of this general type are described and explained in Khouri, US. Patent No. 2,842,647. The base assembly employed in this invention is similar to that described in such patent and includes an electrical impedance element mounted on a base with a rotor pivotally riveted to the base so that its wiper arm bears on such element and is free for rotation with respect thereto. The rotor of this patent has a pair of upstanding ears which project into slots in the knob thereby supporting the knob and connecting it to the rotor.

While this knob is low in cost, which is necessary in this highly competitive field, and adequately shields the rotor, it has been found that it, while ordinarily satisfactory, can be knocked oh the base assembly under certain conditions. This results in exposing the rotor and base assembly and leaves no insulative member which may be manipulated to adjust the variable resistor.

Therefore, the object of this invention is to provide a low cost knob which cannot be inadvertently removed from the base assembly.

Another object is to provide such a knob which protects the rotor assembly against dirt and other foreign matter.

The first object is accomplished by providing a thermoplastic knob having a downwardly depending tongue which passes through the interior of a hollow rivet which holds the rotor to the base. The end of the tongue projects out of the rivet below the base and is enlarged by heat application. After the tongue end is enlarged the knob is free to rotate but is held on the base.

The second object is accomplished by providing the knob with a housing portion which encloses the rotor and includes a flange which bears on the base to prevent ingress of dirt and the like.

Other objects and advantages will be pointed out in, or be apparent from, the specification and claims, as will obvious modifications of the embodiment shown in the drawings in which:

FIG. 1 is a side sectional view of the present knob mounted on a rotor and base assembly;

FIG. 2 is a plan view of the bottom of the knob;

FIG. 3 is a perspective view showing one knob mounted on the base assembly and another knob before it is mounted to the base assembly.

Referring to the drawings, an insulative base panel has an impedance element 12 in the form of a segment of an annulus bonded to it. Conductors 14 and 16 are connected to either end of the impedance element and lead to an edge of the base panel where they are connected to terminals 21. A rotor 22 is pivotally mounted to the plate by a hollow rivet 24 which passes through opening in the base. Rotor 22 has a body portion 26 which electrically contacts rivet 24, and a wiper arm 28 which rides on the impedance element. A central conductor 18 extends from terminal 23 on the edge of the panel and 2 is electrically connected to the rotor through rivet 24 to which conductor 18 is connected at 19.

A pair of ears 30 extend upward from the body portion of the rotor and are spaced to fit in mating slots 32 in a knob 34. The knob is made of thermoplastic material and includes a tongue 36 which extends into and through hollow rivet 24. The end of the tongue is heated and enlarged as shown at 36 to secure the knob to the base panel. This strongly connects the knob to the base and is a quick and inexpensive operation. Rotor ears 30 connect the knob to the rotor so that rotation of the knob rotates the rotor. The knob may be rotated by inserting a screwdriver in slot 29, or a hexagonal wrench may be inserted in hexagonal opening 31, or the top of the knob may be gripped by pliers or fingers or by some other convenient means to effect rotation of the knob.

Knob rotation is limited by the cooperation between shoulder 25 on the knob and car 27 which is fixedly mounted in the rotary path of shoulder 25.

The knob includes a housing portion 38 which surrounds the rotor and impedance element and also includes a downwardly depending flange 40 that rides on the base and keeps dirt out of the inside of the housing.

Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

I claim:

1. In an electrical trimmer having an insulating supporting base and a resistance element fixedly mounted thereon, a conductive rotor pivotally mounted to said base in wiping engagement with said resistance element, a pivot member for said rotor, said pivot member being hollow and extending through said rotor and said base, knob engaging means on said rotor, and an insulative knob including engageable means engaging said knob engaging means, said knob including a tongue extending through said hollow pivot member, the end of said tongue being enlarged to connect said knob to said base.

2. The electrical trimmer according to claim 1 in which said knob and tongue are integrally molded out of a thermoplastic material with the end of said tongue being enlarged by the application of heat.

3. The electrical trimmer according to claim 1 in which said knob includes a housing portion enclosing said rotor, and a downwardly depending flange which bears on said base to prevent ingress of dirt inside said housing.

4. The method of constructing an electrical trimmer comprising, providing an insulative base having an impedance element thereon and an aperture therethrough, pivotally mounting a contact rotor to said base for cooperation with said element by means of a hollow rivet mounted in said aperture, positioning a thermoplastic knob having a depending tongue on said base by inserting said tongue through said rivet, and enlarging the end of said knob projecting through said rivet by application of heat whereby said knob is held on said base, said knob engaging said rotor to cause rotation thereof when said knob is rotated.

References Cited in the file of this patent UNITED STATES PATENTS 2,366,274 Luth et al. Ian. 2, 1945 FOREIGN PATENTS 796,931 Great Britain June 25, 1958 

